Recently compact mobile devices have been widespread worldwide and, at the same time, a demand of a compact, large-capacity nonvolatile memory is rapidly increasing with the significant progress of the high-speed data transfer network. Among others, in a NAND flash memory and a compact hard disk drive (HDD), recording density is rapidly enhanced to create a large market.
On the other hand, some ideas of novel memories have been proposed in order to considerably exceed a limit of the recording density.
For example, a phase-change memory (PRAM) adopts a principle, in which a material that can take two states, that is, an amorphous state (OFF) and a crystalline state (ON) is used as a recording material and data is recorded while the two states are correlated with binary data “0” and “1”.
As to write/erase, for example, the amorphous state is created by applying a large-power pulse to the recording material and the crystalline state is created by applying a small-power pulse to the recording material.
As to read, a small read current is passed through the recording material to an extent that the write/erase does not occur and the electrical resistance of the recording material is measured to read the data. The electrical resistance of the recording material in the amorphous state is larger than that of the recording material in the crystalline state, and the difference in electrical resistance is about 103.
There is also reported a novel memory that has an operating principle very similar to that of the PRAM although the memory is different from the PRAM (For example, see Jpn. Pat. Appln. KOKAI Publication No. 2004-234707).
According to this report, nickel oxide is a typical example of the recording material in which the data is recorded, and the large-power pulse and the small-power pulse are used in the write/erase similarly to the PRAM. In this case, power consumption is advantageously reduced during the write/erase compared with the PRAM.
Although an operating mechanism of this novel memory is not found out up to the present date, reproducibility is confirmed, and the memory is regarded as one of potential candidates for the one with high recording density. Some groups try to find out the operating mechanism of the memory.